1. Field of Invention
This invention relates to a process for producing finely divided aqueous emulsions from materials of limited water solubility. The term "limited water solubility" means a solubility of &gt;.about.10.sub.-5 g/dm.sup.3 H.sub.2 O i.e. greater than about 10-5 grams per cubic decimeter of water.
2. Description of Prior Art
Conventional methods for making fine dispersions in water are contingent upon the use of relatively large quantities of emulsifier and/or very effective homogenisation equipment. The instability of an aqueous emulsion is due to two mechanisms. The drops which come in the neighbourhood of one another will be able to flocculate, that is to say pass over into greater accumulation, something which, if the viscosity is low enough, will be followed by a coalescence, that is an amalgamation. This type of instability will be of particular significance for relatively large drops. With smaller drops another type of instability will be dominant, namely instability made possible by virtue of diffusion. This is due to the solubility in water of a material present in a small drop being greater than for the same material in a larger drop. This effect will exhibit itself the more the smaller the drops are. That is to say that the material which is dispersed will diffuse from small drops over into the large until the small drops have disappeared. This assumes that the dispersed material has a certain solubility so that it will diffuse through the water phase from small drops to larger drops. However, this will be effected rapidly even with materials which have relatively low solubility. With a material with a solubility of about 10.sup.-2 g/dm.sup.3 H.sub.2 O, drops of a size of less than 100 nm (nanometers or millimicrons) will certainly not be able to be produced by homogenisation because they will disappear very quickly by diffusion. It is known that finely divided emulsions can be produced without homogenisation, so-called microemulsions, which are stable, but this demands very large emulsifier concentrations, 20-30% based on the dispersed material against 0.1-2% which is the case in the present invention.
In Norwegian Patent Application No. 76.0554, which discloses a process for the production of emulsions of monomeric vinyl compounds, it is shown that a small portion of a highly water-insoluble material can be homogenised so that finely dispersed drops thereof result and that afterwards the monomer of limited water solubility can be stirred in with weak agitation. The monomer of limited water solubility then diffuses into the small amount but higher number of drops of the water-insoluble material with the result that there is formed a stable, finely dispersed emulsion of the monomer of limited water solubility. Degradation by diffusion is, in this case, prevented by the presence of the water-insoluble material which on diffusion will create an osmotic counter-pressure. This process of making stable emulsions of monomers of limited water solubility is advantageous since it reduces the need for homogenisation because only a small portion of a water-insoluble material is homogenised. However, the process requires homogenisation of the small amount of insoluble material. In order to obtain sufficient fine division, a very intense homogenisation is required and it will, at any rate, be difficult to arrive at below 200 nm. in diameter for the drops.
Norwegian Patent Application No. 76 0554 thus deals with a process in which a finely divided emulsion of a water-insoluble material has been pre-prepared and this is utilised to make a dispersion of a far greater quantity of a material of limited water solubility due to the latter material being diffused into the pre-formed drops of the water-insoluble material.
Particles having a very low and controlled size are produced by the well-known method of emulsion polymerisation whereby a vinyl compound is polymerised in the presence of an emulsifier in water. By this process, readily formed particles having a controllable size of 10-500 nm. are obtained. These particles are composed of polymers and will only be able to take up 0.5-5 times their own weight of the material of limited water solubility.